1. Field of the Invention
This invention pertains, in general, to passive direction and range finding, and in particular, to a method and apparatus for passive ranging from a moving platform to a stationary, non-cooperative emitter.
2. Description of the Related Art The survivability and military effectiveness of low altitude aircraft and surface craft are often strongly dependent upon the ability to locate hostile radar-directed defense weapons quickly and accurately for purposes of their avoidance, evasion, suppression and/or destruction.
There are a variety of methods of passive location of stationary, ground-based emitters from moving platforms, the most common techniques being the azimuth/elevation (AZ/EL) method, conventional triangulation, and the multiple-aircraft time-difference-of-arrival (TDOA) method. These methods may be further subdivided into techniques providing for direction of arrival (DOA) measurements only and those providing both DOA information and range information.
Currently-employed techniques for measurement of DOA comprise either Amplitude Comparison techniques or Phase Interferometry techniques. The former typically consists of four broadbands, orthogonal antennas covering 360.degree. in which a comparison of amplitudes of incoming signals between adjacent antennas provides DOA information. These provide accuracies of from 3.degree. to 10.degree.. (See A. R. Baron, et al., "Passive Direction Finding and Signal Location", Microwave Journal, September 1982, pp. 59-76.)
Phase interferometry techniques (in their simplest form) utilize a pair of antennas disposed on the moving platform and spaced apart by a known distance such that a plane wave arriving at an angle relative to the pair is received by one antenna earlier than the other, due to the difference in path length traversed by the wave. If the signals from the two antennas are processed, their phase difference provides an indirect measurement of DOA relative to the antenna pair. Interferometer DOA accuracy is a function of antenna spacing and azimuth and elevation angles, and systems have been built having operational accuracies of from 0.1.degree. to 1.degree. RMS. Interferometers having more than two elements are also known in the art.
In terms of emitter location techniques, TDOA methods are the most accurate, but require a multi-platform (typically 3) system to range on a single emitter, in which the time-of-arrival differences at receivers on the platforms is measured and processed in conjunction with the known position of the platforms to localize the emitter. Since the technique entails multiple platforms and complicated distance-measuring and timing equipment, it is not considered as a suitable ranging method for single moving platform systems.
AZ/EL techniques locate an emitter by measuring the azimuth and elevation angles of arrival of the signal and the altitude of the platform relative to the ground. The slant range is then computed from trigonometric relationships assuming the emitter is also at ground level. The range error is a strong function of target range and altitude, and all other things being equal, provides better accuracy only at large altitudes of platform above the emitter. Significant measurement errors can also result unless external information is available to the system concerning emitter altitude due to topography.
Typical AZ/EL location systems utilize a pair of orthogonal phase interferometers to obtain azimuth and elevation angle information.
Triangulation techniques employ two or more DOA measurements made at subsequent times as the platform traverses its path of motion relative to the emitter and compute range using well-known trigonometric relationships.
The triangulation method may employ either Amplitude Comparison methods or interferometry to measure DOA information. However, since interferometry may provide as much as 10-to-1 improvement in accuracy, interferometry may become the preferred method where system accuracy demands are high, even at the expense of increased system complexity.
This, it is possible to predicate an extremely-precise, passive ranging system for a single moving platform which utilizes either the interferometer-based AZ/EL method or the interferometer-based triangulation method. However, both techniques suffer from certain problems which limit their practical application. First, they require extremely accurate navigation information, particularly platform heading. As indicated above, AZ/EL systems are highly dependent on platform altitude and emitter range and are highly susceptible to terrain-induced errors. Triangulation methods are relatively slow and are dependent upon true bearing spread, i.e., the angle subtended at the emitter by the path of the moving platform between measurements. Also, triangulation becomes ineffectually-inaccurate at small intercept angles, i.e., large angles relative to the interferometer's boresight. Finally, since triangulation requires that the system be able to collect data on the emitter's signal for many seconds, its performance is degraded by intermittent signals, i.e., if the emitter is scanning, or at low platform altitudes in mountainous terrain, signal intercepts may be sporadic, thus reducing system effectiveness.